setFieldAttribute and Subform

    public function getForm($data = array(), $loadData = true)
    {
        // Get the form.
        $form = $this->loadForm('com_evisit.visit', 'visitform', array(
                        'control'   => 'jform',
                        'load_data' => $loadData
                )
        );

        if (empty($form))
        {
                return false;
        }

       $form->setFieldAttribute('myfield', 'required', 'true');

        return $form;
    }